Presentation on theme: "NOTES: 17.1 – Thermochemistry – The Flow of Energy"— Presentation transcript:
1 NOTES: 17.1 – Thermochemistry – The Flow of Energy
2 Thermochemistry● THERMOCHEMISTRY: the study of energy (in the form of heat) changes that accompany physical & chemical changes● Heat flows from high to low (hot to cool)
3 Energy (E): ● measures the capacity to do work or supply heat ● energy is weightless, odorless, and tasteless● can only be detected because of its effects
4 Energy (E):● energy stored within the structural units (i.e. molecules) of chemical substances is called: CHEMICAL POTENTIAL ENERGY (PE)EXAMPLE: gasoline contains a large amount of chemical PE; when the gasoline burns in your car engine, the energy is released to do mechanical work
5 Heat (q):● measure of the total amount of energy (E) transferred from an object with high energy to an object with low energy● cannot be detected by the senses or instruments…only changes caused by heat can be detected
6 Magnitude of Heat Flow 1 kJ = 1,000 J 1 calorie = 4.184 J ● Units of heat energy:1 kcal = 1,000 cal = 1 Cal (nutritional)1 kJ = 1,000 J1 calorie = J1 kcal = kJ
7 Heat (q): ● heat always flows from a warmer object to a cooler object EXAMPLE: when you put an ice cube into a cup of hot water, heat will flow from the hot water to the ice cube… the ice melts and the water’s temp. decreases
8 Temperature (T): ● Measure of average kinetic energy ● Indicates relative amounts of energy● Indicates the hotness or coldness of an object
9 Thermochemistry● all chemical reactions and changes in physical state (melting, freezing, etc.) involve either the release or absorption of HEAT● in thermochemistry we examine the flow of heat from a “system” (i.e. a chemical reaction) to its surroundings (or vice versa)
10 How does energy change? ● Law of Conservation of Energy: -In any chemical reaction or physical process, energy is neither created nor destroyed;-all of the energy involved in a process can be accounted for as: work, stored energy, or heat.
11 Thermochemistry● exothermic reactions: release energy in the form of heat; show a negative value for quantity of heat (q < 0)● endothermic reactions: absorb energy in the form of heat; show a positive value for quantity of heat (q > 0)
12 EXOTHERMIC CHANGE: ● losing energy ● products have less energy than the reactants● ΔH = (-)
13 ENDOTHERMIC CHANGE: ● gaining energy ● products have more energy then reactants● Δ H = (+)
15 Endothermic or Exothermic? ● H2O (l) H2O (s)Exothermic(liquid water freezing to solid ice)
16 Endothermic or Exothermic? ● Melting - solid to liquid● Boiling - liquid to gas● Condensing - gas to liquid● Freezing - liquid to solid● Sublimation - solid to gas
17 Endothermic or Exothermic? ● Melting - solid to liquid endothermic● Boiling - liquid to gas endothermic● Condensing - gas to liquid exothermic● Freezing - liquid to solid exothermic● Sublimation - solid to gas endothermic
18 In conclusion… ● Solid Liquid Gas (endothermic) ● Gas Liquid Solid (exothermic)
19 Cup of water at 30o C vs. tub full of water at 28o C: ● Which has a higher kinetic energy?● If the cup of water was poured into the tub of water, would there be a transfer of heat at that moment? Would the temperature change?
20 Heating and Cooling Curves: ● When a plot of temperature vs. time is plotted as a pure substance is heated or cooled, a plateau is observed at the substance’s melting and boiling points.
21 Heating and Cooling Curves: ● In other words, as energy is added or removed from a substance, there is no change in temperature at the pure substance’s melting and boiling points until it has completely melted or boiled.● mmkay